allow specifying a custom rendering pixel size

[restaste]

Hi 👋

We've been using morecantile successfully for a while now and we started using it to connect to ArcGIS Online services, which generates custom tiling schemes.

Doing so, we noticed ArcGIS servers give a tileInfo.dpi value (with a value of 96 in this case). Converting this to mm gives 0.264mm, which we figured we should use instead of 0.28mm as the rendering pixel size. Since this number is used to compute scaleDenominator, resolution and finally tile coordinates, our reasoning was that getting this number right mattered. So we forked morecantile to extend it to support a custom rendering_pixel_size parameter.

The OGC specs do mention this 96 DPI vs 0.28mm as a potential source of confusion, and digging further into how tile coordinates are computed in morecantile, it seems like rendering_pixel_size cancels itself out in the math?

• scaleDenominator is computed as scaleDenominator = resolution * meters_per_unit / rendering_pixel_size, where resolution is defined as (bbox_width_meters / tile_width / 2**zoom_level)
• To generate tile coordinates, resolution is later recomputed from the scaleDenominator as resolution = scaleDenominator * rendering_pixel_size / meters_per_unit.
• This simplifies to the original resolution = bbox_width_meters / tile_width / 2**zoom_level, where the rendering pixel size doesn't appear anymore.

I'm probably misunderstanding some important bits, but:

• Is my understanding correct that changing rendering_pixel_size to some value other than 0.28 does not impact the tile coordinates computations?
• Would it have other (positive or negative) side-effects?
• Is this ArcGIS DPI value safe to ignore? We'll have to ask the folks at ESRI, but I wondered if you had an idea what other purpose this value could have (some purely rendering concerns?)

Disclaimer: this is out of my comfort zone, happy to learn more!

[vincentsarago]

renderingPixelSize is not part of the OGC specification for the TileMatrix object but we could resolve it by using `private variable like https://github.com/developmentseed/morecantile/blob/main/morecantile/models.py#L384-L387

[vincentsarago]

👍

[vincentsarago]

NOTE: Since the 1980s, the Microsoft Windows operating system has set their default standard display pixels per inch (PPI) to 96. This value results in an approximated 0.264 mm per pixel. The similarity of this value with the actual 0.28 mm adopted in this standard can create some confusion.

Microsoft Windows and ESRI together 🤝 😅

Thanks for this great explanation and PR @restaste 🙏

This simplifies to the original resolution = bbox_width_meters / tile_width / 2**zoom_level, where the rendering pixel size doesn't appear anymore.

yeah, makes absolute sense BUT we don't have access to the bbox_with_meters variable once the TMS is created

NOTE 3 The bounding box of a tile matrix is not supplied explicitly because it can be calculated from cornerOfOrigin, pointOfOrigin, tileWidth, tileHeight and scaleDenominator.

😓 🕳️

I don't think we can have renderingPixelSize at the TileMatrix level mostly because it does not respect the OGC TMS specification. So I don't think there is an easy way to have this value both at TMS creation time and at usage time :-(

To enable custom implementation we could maybe have an environment variable which will be use in Morecantile (but this is custom and won't enable sharing TMS document (because there is no way for a client to know if values used was 0.28 or 0.26

At the same time (if we can't make ESRI Change) you could open an issue in https://github.com/opengeospatial/2D-Tile-Matrix-Set/tree/master to see it we can add screenPixelSize in the TMS Spec (at tileMatrixSet or tileMatrix level)

[vincentsarago]

For the purposes of this International Standard, the common pixel size is defined to be 0,28 mm × 0,28 mm. Because arbitrary clients can request maps from a server, the true pixel size of the final rendering device is unknown to the server.

WTF Esri is not using 0.28 😓

[vincentsarago]

I think there IS a solution, the resolution for the tileMatrix is the cellSize 😅

ref opengeospatial/2D-Tile-Matrix-Set#4

[restaste]

Closing this one since #140 was merged.

Thanks @vincentsarago !